To be effective, and often to comply with local regulations, centrally located alarm switching systems such as fire alarms or burglar alarms require that the signal path integrity of connected speakers and their associated wiring be monitored regularly. Similarly, the wires that connect alarm initiating devices to the central switching location such as a fire alarm pull stations or smoke detectors must be regularly monitored. For this reason, the speakers and alarm initiating devices for alarm systems are generally wired separately from the speakers for intercom systems.
Typically, path integrity for the wiring and the speakers is verified by monitoring for wiring faults, specifically short circuits, open circuits, or grounded wiring or speaker connections. Prior art systems such as U.S. Pat. No. 3,849,883, U.S. Pat. No. 3,912,883 and U.S. Pat. No. 4,723,292, attempt to monitor speaker wire integrity by supplying a direct current on the speaker wires, blocking the direct current from flowing through the speaker coils, (generally with a series capacitor), but providing a resistor in parallel with the speaker coils to allow DC current flow. By evaluating this direct current flow, the wire pair can be tested for shorts or open circuits. However, these type of systems are unable to detect open speaker coils or problems in the connection between the speakers and the wire pair, since the parallel resistor will conduct the same amount of direct current regardless of whether the speaker coil is open or if the speaker coil is not even connected.
Other systems supervise path integrity by transmitting a signal carrying a subsonic or supersonic frequency through the audio pair of wires, and then separately picking up the signal near the speaker. For example, a dual cone system is used wherein one speaker generates the inaudible frequency and the other picks up this frequency. A corresponding signal is then separately returned on another wire pair so that the frequency and amplitude of the returned signal can be analyzed to determine if there are problems in the speaker or the path. However, this system is costly both in terms of parts and installation, as it requires a signal generator, some type of pick-up device, another pair of wires strung to the speaker location for returning the picked-up signal, and frequency detection means.
Moreover, these inaudible frequencies can cause problems with pets that are more frequency sensitive than humans, and are even reputed to cause problems with certain people. Consequently, the problems associated with broadcasting nearly audible frequencies, along with the extra expense of four wire systems and associated pick-ups, signal generators and detectors, render these systems less than desirable. Other frequencies further from the audible range are not used because these frequencies cause problems with the speaker design and bandwidth, and can create electronic interference.
As a result of the problems with frequency based testing, another type of testing is performed, where the alarm speakers are temporarily disconnected from their driving source and tested with a DC current. The current level through the circuit is evaluated to determine integrity of the signal path. However, this type of monitoring is not useable with intercom, public address, music, or paging type of systems because frequent interruption of the system is necessary. Indeed, some standards require that the integrity of each speaker and signal path be tested at least every 30 seconds. Even if elaborate switching electronics were designed to independently isolate and scan each speaker individually, the speakers would be interrupted at least every 30 seconds and therefore could not be employed satisfactorily as intercom, paging, or music speakers.